Recently microblogging social giant put enourmus efforts to improve they direct messaging system. This in fact is second try. In 2013 Twitter allowed sending DMs to anyone on Twitter, not just to the followers. As soon as this feature went life, spammers took advantage of this new feature which brought direct messaging feature completetly down for some time. Later one DM functionality was restored, but users were not able to pass links inside the DM, and were advised to open request for twitter support to whilelist their URLs. Also many known shortening services, like bit.ly were completely blocked on twitter. It took almost a year to reenable links via twitter.com, but twitter APIs still were not able to send links in the DMs, receiving famous error 226. This error was even not documented on developers site on twitter, which raised many complaints across developers. Twitter support was silent, or giving meaningless answers. Many services like SocialOomph and our Twitter Auto DM advices their customers about these limitations and what can be done to bypass them, like sending link as clear text by adding slash infront of link: /www.vicconsult.com

On Twitter official support website they put banner stating that they are restructuring their direct messaging system and soon it will be restored.

We're restructuring back-end elements of our direct message system. As a result, users may be unable to send some URLs in direct messages. We apologize for the inconvenience.

That banner stay for almost 2 years. Also developers site was updated with more explanations about 226 error code.

Thanksfully on April 2015 the service restored completely. Twitter again allowed sending DMs to anyone, but this time intrduced new setting parameter which enables receiving DMs from anyone, and by default it is off. Not sure how many people will actualy turn it on.

Another big change was on Twitter's support website where they explain best bractices for automation. Previously Twitter had statement that sending auto DMs is not advvices and can be considered as spam. Later in April 8th 2015 it was updated as follows:

With express user consent, automation of Direct Messaging is permitted.

Prior to April 2015 users were able to send 250 DMs a day, manualy or via APIs. On April 2015 this rate was increased to 1000 DMs a day.

On June 2015 Twitter announced that they will remove DM size limitation of 140 characters, making it literely unlimited:

We’ve done a lot to improve Direct Messages over the past year and have much more exciting work on the horizon. One change coming in July that we want to make you aware of now (and first!) is the removal of the 140 character limit in Direct Messages. In order to make this change as seamless as possible for you we’ve included some recommendations below to ensure all your applications and services can handle these longer format messages before we flip the switch.

All this will bring more attention from Twitter users to utilize more direct messaging. But as we know along with all these positive directions from Twitter, spammers will try to get as much as possible from these new features.